US2427960A

US2427960A - Extrusion machine

Info

Publication number: US2427960A
Application number: US492851A
Authority: US
Inventors: Griffiths Francis Tadman
Original assignee: WT Henleys Telegraph Works Co Ltd
Current assignee: WT Henleys Telegraph Works Co Ltd
Priority date: 1942-07-31
Filing date: 1943-06-30
Publication date: 1947-09-23
Anticipated expiration: 1964-09-23
Also published as: GB559153A

Description

Sept. 23, 1947. F. T. GRIFFITHS EXTRUSION MACHINE Filed June 30, 1943 5 Sheets-Sheet l m mm wm Nm om u vm om Inventor: Franc /5 TGr/'ffihs Hl-S Attorneys Sept. 23, 1947. F. T. GRlFFlTHs 2,427,960

i EXTRUSION MACHINE l Filed June so, 1945 5 sheets-sheet 2 Inventor.'

Franc/'s 7. Griffin/ls His Attorney,-

Sept. 23, 1947. F. T. GRlFFlTl-ls 2,427,960

EXTRUSION .MACHINE Filed June 50, 1943 5 Sheets-Sheet 3 F/GZ.

,.20 z 'l l Inventor:

Francis Z- C/rl'ffi H13 By rz/ehm, 1 M

His A ltorneys F. T. GRIFFITHS EXTRUsIoN MACHINE Sept 23, 1947.

5 Sheets-Sheet 4 Filed June 30, 1943 Franc is T Uri/'finas His Attorneys Sept. 23, 1947. F. T. sRlFFn'Hs EXTRUSION MACHINE Filed June 50, 1945 5 Sheets-Sheet 5 Fran C111,` TG

:surfaces is circular. 'rthe innerand outer members are so formed, by -lproviding on at least one surface helically eX- `tending projections and/r recesses, that relative rotary movement of the two members causes material fed into one end of the annular space be-v V:tween them to be feel forward. Continuous relative rotary movement of the members constitut- Patented Sept. 23, 1947 l UNITED STATES PATENT l()FFI-CE EXTRUSION MACHINE Francis Tadman Griffiths, Gravesend, Kent, England, assignor to W. T. lillenleysy Telegraph Works Company, Limited, Dorking, -'Surrey, England, a British company Application J une 30, 1943, Serial No. 492,851

In Great Britain July 31, 1942 (Cl. lf3- 12) 8 Claims. 1

This invention is Vconcerned with extrusion ma- Achimes of the kind in which material in a highly -viscous or plastic state is kdriven forward into an die vby means of a screw thread impelling device.

"The 4impelling device usually comprises two longitudinally extending concentric members of fwhich the general cross-sectional shape of the The adjacent surfaces on ing the impelling device, combined with continuous'feeding of the material, results in the con- 'stinuous delivery of the material in the plastic 'state to the extrusion chamber and in extrusion of it through the die or between the inner and vvouter die, as the case may be, by the pressure exerted on it bythe following material.

"Under normal conditions the output of an eX- trusion machine kof the kind described appears to rbe dependent among other factors upon the 'pressure prevailing at the `delivery end of the imfpelling device, an increase in the pressure resultfing in .an increase in output and, conversely, a

decrease in pressure resulting in a decrease in output. Thepressure 'that is necessary in order "to extrude material through the extrusion orice lis-considerable (ofthe order of ten to twelve tons per square inch where the material is lead or a vlead alloy). It follows, therefore, that there will -be vexerted a correspondingly great reaction on the member (or members) furnished with the lscrew vthread projections and/0r recesses, which reaction will have a considerable component in va-direction parallel to the aXis of the impelling device and will tend to move the member longitudinally. lI-llitherto it has been considered necessary to anchor the member against such movement either by rigidly securing it to a support,

generally the machine casing, or, where the memfber `is rotatably driven, by inserting one or more heavy thrust bearings between it and a support,

4l-generally the casing. In accordance with the present invention, however, I allow one of the Vtwo relatively V rotatable parts constituting the limpelling device, that is subject to axial thrust cio Cil

in the thrust exerted'uponitand utilise this axia movement to adjust the output of the device automatically in a direction tendinglto restore it to normal. It will vbe'apparent that, 'to maintain this permitted axial movement 'within the predetermined positive limits 'between which "there is no positive restraifnt, 'smercounter pressure is necessary. In 'a fs'in'g'le ilfnp'ellertype extrusion machine, 'this -r'nay b'e 'fafs'p'ring force Vopposing the component in an Yaxial direction of the pressure exerted*onthe-'aXially movableinember by the plastic 'materialaltthedelivery 'end of the impelling device. 'In -an extrusion. machine comprising duplicateinipelling devices, the' counter pressure opposing-ith'e'faxial 'thrust 'exerted by the plastic material fon the axially movable .member of one impelling device may be provided wholly orin part bytheiaxial thrust "eXe'rte'd'on the corresponding'member 'of-the otherimpelling device, which' is coupled to it.

There are severaliwaysfin'which'the output of the impelling vdevice "(or `of each device) of an extrusion machine xm'ay v1be controlled automatically by a permitted"axialmovement of one of the two'relatively rotatablemmbers constituting the device. iIn :thecase of 'a-machine for the extrusion of plastic metal, the'permitted laxial movement of one-1 ofthe two `parts 'of -an impelling device may be tsut:h"'a=s"`to produce a relative longitudinal displacement ofthe two parts of the device such that theefectivelength of thedevice is changed and Vhence the pressure an-d output of that device correspondingly Aincreased or diminished, as the 'casermay be. `rlrlternatively or in addition, 'theperm-itted axial movement vvof one of theftwo parts 'of lanimpellingdevice may be employed to operate an electric switch or `a valve controlling 'thesupply of h'eatto, or the abstraction of heat from, van appropriate part or parts of the impelling'device. 'For'instance, axial movement of the'rotatablyA driven part of animpelling device relative 'to the co-'operating fixed part due to an ex'ce'ssipressure'at-the'delivery Vend of the device may automatically increase the amount oi heatfappliedtothe forward end of the device and also, or alternatively, decrease the amount of heat abstracted vfrom 'thefeed en'd yof the device, so reducingftliepressure at the delivery end.

In the case of an extrusion'mafchine'for a material such as rubber'or a-synthetic'pla-stic, control oi the output by axialrmovement of a part of the impellingdevice'iniresponse to a variation in pressure at the delivery yendlof it may be'obtained by using this movement `to Vcontrol the rate at which stock is fed in, either by adjusting the speed of an automatic feed device or by varying the width of the stock being fed in.

Automatic regulation of output in accordance with my invention is especially valuable in the case of an` extrusion machine with a pair of impelling devices forcing material into a common extrusion ch'amber and out through a common die. With such machines diiiculty may be experienced in maintaining the correct pressure at the delivery end of each impelling device. Failure to do this will result in uneven delivery to the extrusion chamber and in a non-uniform product, for instance, a tube with wall thickness that is not uniform, a defect that cannot readily be detected without cutting the product. By applying the present invention to such an extrusion machine any deviation from the normal of, or any out of balance between, the pressure at the delivery ends can be rectified automatically before it reaches a value detrimental to the product.

By way of example and in order that the invention may be more fully understood, some ways in which it may be applied to extrusion machines of variouskinds will now be described with the aid of the accompanying drawings wherein,

Figure 1 is a. longitudinal section of a lead extrusion machine for the production of lead tubing,

comprisingtwo pairs of impelling devices,

Figure 2- is Va cross-section of the machine shown in Figure 1, the section being taken along Vthe line II-IIof that figure,

Figure 3 is a longitudinal section of a lead extrusion machine comprising a single pair of axially aligned impelling devices, and

Figure 4 is a longitudinal section of a lead trusion machine of the type in which a single impelling device is used comprising a rotatably driven sleeve carrying. screw threads on its invternal and external surfaces.

Figures 1 and 2 of the drawings, to which reference will first be made, show a lead extrusion 1 machine having two pairs of axially aligned screw thread impelling devices, the rotating parts of each pair of which are mechanically coupled together at their delivery ends, disposed on diametrically opposite sides of a central extrusion Y chamber and with their axes parallel to one another and at right angles to that of the extrusion chamber, as described and claimed in my Patent 2,367,394. The machine casing I is in the form of a'pair of barrels 8 united by a central.

'the enlarged portion of the aperture and is held in' place by a tubular nut 4. The other or rear end of the aperture receives a tubular nut 5 constituting a holder for an inner die or point 6 which co-operates' with the matrix 5 to form an annular extrusion orifice 1. Both ends of each barrel 8 are counter bored to receive sleeves 9, the inner end parts of the internal surfaces of which are provided with screw threads I0 and the outer end parts of the internal surfaces of which are circumferentially grooved to form sealing glands and bearingsr II fora rotatably driven impeller 'shaft I2 which extends'the whole length ofthe barrel andprojects well beyond the casing at one end thereof.V Each' impeller shaft I2 carries a pair of screw 'threads I 3 of opposite hand which co-operate with the threads I9 on the two sleeves and with them constitute a substantially thrustbalanced pair of impelling devices. These are fed with molten metal at their outer ends, each from an annular feed chamber I4 which is located in the wall of the sleeve 9 and fed through a feed pipe (not shown). In the regions of the feed chambers I4 it is necessary to apply heat to maintain the metal in a molten state until it is engaged by the impelling surfaces I0 and I3. In the particular example of machine shown in the drawing, this is effected by means of electric heaters I6 inserted in the walls of these parts of the sleeve, but it will be appreciated that other forms of heaters may be used instead. To ensure rapid cooling of the metal to a plastic state when between the impelling surfaces, it is necessary to extract heat from the central part of each impelling device. This is done by circulating a cooling medium, generally water, through passages I'I in the walls of the barrels 8 or through channels I8 in the external surface of the barrel which are closed by a surrounding collar, but again, other methods of cooling are applicable. Under suitable temperature conditions, rotation of the 'shafts I2 will cause molten metal fed into the impelling devices and there cooled to be impelled forward by the several devices into annular delivery chambers I9 and th'en through ports 253 into the common extrusion chamber 2. To ensure -satisfactory flow of plastic metal through the delivery chambers I9, ports 2li and extrusion chamber 2, it will also generally be necessary to apply heat in the regions of the delivery ends'of the impelling devices and in the region of' the ports and extrusion chamber. In the particular machine shown, this is done by the provision of electric heaters 2I and 22 in the wall of the casing. The hottest parts of the casing will naturally be those parts in the regions of the feed chambers I4 and the coolest, the parts 23 surrounding the middle parts of the impelling devices. Between each cooled part of the casing and the adjacent feed chamber the cross-sectional area of the casing is substantially reduced to reduce the flow of heat between these parts, in accordance with the invention described and claimed in my cepending application, Serial No. 492,852, filed June 30, 1943, for Extrusion machines. This reduction is effected by terminating the end of the barrel a short distance from the adjacent external wall of each feed chamber and holding the sleeve in place in the barrel by means of a collar 24 which engages, over a small area only, an annular face 25 on the sleeve and is drawn towards the barrel by bolts 2S. The part of each impeller shaft l2 which projects beyond the casing I is supported in a pair of bearings 3i) housed in pedestals 3|. The upper of the two impeller shafts extends beyond its outer bearing 3E! and carries a driving wheel 32. Between its two bearings 3B this upper impeller shaft also carries a gear wheel 33 which engages with a correspondingly positioned gear wheel 33 on the lower impeller shaft so that the latter is driven at the same speed as the upper shaft. The wheels 33 are each secured against longitudinal movement on the shaft by means of a pair of collars 34 which are screwed on the shaft one on each side of the wheel 33. In accordance with the present invention, each impeller shaft I2 is permitted to move axially relative to the casing l and the bearings 30 to a limited extent without positive restraint under the influence of change in thrust exerted upon it. The extent to which the impeller shafts I2 are permitted to float axially is determined lby the clearances between the end ,face 35 of Aeach of the collars 34 andthe adjacent vface -31 of a ball thrust washer 35secured to the adjacent end face ofthe neighbouring bearing support. These clearances are preferably such that the vtotal permitted axial movement between the two limits imposed by the ball thrust washers 35 is of the order of one pitch the effective length of the impeller thread I3 of A the left hand-device (since a part that was effective will move into a region nearer the feed chamber where it is ineffective) and moving the delivery point to the left and thus further -from the port 20 leading to the extrusion chamber 2. The eiTect will .thus be Vto diminish the pressure in the metal at this port and, therefore, the vrate of delivery through the port as compared with that of the right hand impelling device, the delivery from which will be increased, though not necessarily to the same extent, because the delivery point will Vhave approached the .right hand port. When a balance of pressure has been obtained, axial movement of the impeller shaft will cease and uniform output from the `two upper impelling devices will prevail until an alteration in conditions, for instance, in the Vtemperature of the metal fed to one of these two devices, produces a diierence in pressure and output which will be corrected by movement of the floating 5 member either in the same or in the reverse direc- It may not always be possible to obtain unii' formity of output from each of two aligned pairs of impelling devices merely by allowing Vthe impeller shaft to iioat axially, for the extent to which it may float is limited by practical considerations, and, as indicated above, we prefer,

either in addition or as an alternative, to provide for automatic correction of the respective outputs of the two devices of each pair by arranging for axial movement of the impeller shaft to operate an electric switch or a valve controlling the supply of heat to, or abstraction of heat from, an appropriate part of one or both of the impelling devices. To this end, .each impeller shaft of the machineshown in Figures 1 .and 2 of the drawings carries a circumferentially grooved collar 38 located between the machine vcasing and the adjacent bearing 30. The support for this bearing carries a bracket V39 with a forked end supporting a pivot pin 40 passing through a rod 4I, the lower vend of which carries a -roller which ff engages in -the groove in the collar 38. The upper end of the rod 4I carries a toothed quadrant 42 which engages a pinion 43 rotation of which operates a temperature control device 44. This `device 44 may incorporate a rheostat to lcontrol the iiow of current through the heaters -ZI and certain of the heaters V22, in the region of the delivery end of one ofthe two impelling devices by wh'ich it is actuated, or a valve for vcontrolling .the flow of cooling iiuid through thechannels ISO 1J around one :device. Preferably, however, the @device is arranged :to `operate diierentially `on l:both-impelling.devices ofithepair. v` It may, therefore, comprise `a -doublefrheostat 'so that, as lthe amount of heat .applied to the forward/end vof one -fdevice fis-*changed in Vone direction, the amount 'of .heat applied to `the iforward ,end-of the other device .is correspondingly changed in' the reverse direction. In this case, vaxial movement of fthe impeller ,screw in :a direction rearwards (i. e. Vtowards the :feed end) relative to the 'device deliveringmore metal 'will operateto increase the amount `of heat supplied to the Aforward end of f that device and reduce the amount Ysupplied'to the other, thereby increasing the Aslip or kshear in the "metal iin the first device and'hence reducing the fslip .or :shear .in the `other device, .and .Shen-ce reducingzthefoutput of the iirst device and lincreasing `the output ofthe other device. vAlternatively .or in addition, the control .device '44 may. :comprisea pairo'f `Vregulating valves inserted in the pipes :conveying water yor other cooling 'fluid to the cooling region of each impelling device of the pair actuating the control device and arranging th'atlthe `valve stems of these are nad- 'justedsimultaneously but inopposite and appro- .priate directions by 4rotation of .the pinion 43. With :such .an arrangement, in the 'event of an excess pressure at the delivery lend of -one .impelling :device causing 'that device to .fdelivermo're metalthan the other,'the impeller shaft will vmove rearwards 4relative to the iirst device, rotate the :pinion 43 Yand'adjusttheisetting of the Atwo valves vin such Aa wayras to reduce the flow of vcooling fluid vto th'e rst ydevice and increase the Yflow vto ithe vother device.

Figure -3 shows how the linventionmayfbe ap- .plied to :a Fbala-nce lead extrusion machinefhaving 4-a'casingl which houses'or forms the outer concentric parts 52 of two axially aligned screw thread impelling devices of whichthe vinner .parts 53 are formed by, or carriedon, a tubular stationary member 54 about which th'e Acasing rotates. The casing is supported vin a central bearing 55 and on the stationary inner member -5'4 each end .of which 1projects beyond the casing and is :fengaged by .a support 56. The extrusion -chamber 15'! and its inner and outer dies, 58 and 59 respectively, areco-a-xial with the impelling devices and are -located within the central part of the stationarymember 54, the chamber being of annular -form and -fed with -plastic Ilead th'rough a number of ports-5,0 from an annulardelivery chamber 5I common .to `both impelling devices. In this machine, temperature control of the impelling devices is eiected by heating the extremities of the rotating :outer vcasing 5'I -by 4burners S2-and cooling lthe rneighbouring parts by jets -83 yof air .or water. Here each heated part fis heat isolated from the neighbouring cooled part -by'a circumferential groove 64 in 'the external Isurface of the rotating casingpas described andclaimed in :my co-pending application, Ser. No. 492,852. Material in va liquid .state 'ris fed to the opposite ends of the two impelling devices through ports in the wall ofthe stationary member, the vports themselves Abeing supplied 'each Vthrough a pipe :66 -passing through the 'wall of the support 56 and preferably yheat insulated therefrom. The invention :is applied by allowing-the rotating Youter casing 5| :to float endwise toa limited extent on the inner supporting member l54 by Vproviding an .appropriate clearance between each .end face :of the Acasing '5I .and the adjacent face A(il of one of .a pair of -ball thrust washers 68 for which the supports 56 form fixed abutments. and abstraction of heat are controlled by axial The supply movement of the casing in much the same way as described with reference to Figures 1 and 2, in that axial movement of the casing 5I rocks a pivoted lever engaging ina circumferential groove in the casing. In this case however, the free end of the lever carries a cross arm l0, the ends of which are forked and engage each a grooved collar on the stem of one of two valves 'Il controlling the supply of water to be sprayed on the'outer rotating surface of the casing. It will be seen that movement of the casing in one direction will open one valve and close the other. Alternatively or in addition, valves 12 may be inserted in the supply of gas or other fuel to the burners 62 and controlled differentially by cross arms 13 by the rocking of the lever 69. With this arrangement, movement of the lever in one angular direction will reduce the opening of one valve and increase that of the other, and angular movement of the lever 69 in the converse direction will operate the fuel valves in the reverse direction.

Figure 4 shows the way in which the invention may be applied to an unbalanced form of ex; trusion machine which, in this particular example, comprises a tubular impelling member 8B, which operates in an annular chamber formed between the machine casing 8l and a stationary tubular member 82. The member 80 is provided with screw threads 83 and 84 on its internal and external surfaces respectively. These threads cooperate with surfaces on the walls of the annular chamber to form a pair of concentric impelling devices for impelling plastic metal into an extrusion chamber 85 and thence between an inner die or point 8B carried by the member 82 and an outer die 81 in the front wall of the casing. The tubular impeller member 80 extends rear- Wardly beyond the casing and is driven by means of a gear wheel 88, the heavy thrust on this member being taken up by means of a, massive thrust bearing 89 inserted between a circumferential collar 90 on the rotating member and the rear wall of the casing. Heat is applied to the impelling device in the region of the feed chamber 9| and also towards the delivery end of the device by a pair of induction heating coils 92 and 93, but naturally other methods of heating may In applying the invention to this form of machine, I prefer to position the thrust collar 90 on the tubular member well forward of the thrust bearing 89 and to insert between the two parts a number of helical springs 94 which are compressed to an extent depending upon the pressure exerted by the plastic metal on the threads and front end face of the rotating member. With this arrangement it will be seen that the effective length of the impelling device is controlled by the pressure in the plastic metal at the front end of the machine. Thus, if for any reason the pressure increases above normal, the effective length of the impelling device will be reduced and the pressure correspondingly reduced and vice versa. In addition, I may use this limited axial movement of the rotating member 8i! to control the supply or current to either or both of the induction heating coils 92 and 93 so as to control the amount of applied heat in accordance with the pressure exerted on the rotating member, as shown diagrammatically in the drawing.

Each of the extrusion machines described 8 with reference to the drawings is shown as furnished with a particular form of arrangement for applying heat to or abstracting heat from the impelling device or devices and a form of control appropriateto the particular heating or cooling means used has been described. It is to be understood, however, that these different forms of heating or cooling means can, in most cases, be used on machines other than of the type on which they are shown. For instance, induction heating coils may be employed on the machine shown in Figures 1 and 3 instead of electric resistance heaters and burners, and resistance heaters can be employed on the machines shown in Figures 3 and 4, although in the former case current collector rings might be necessary.

V What I claim as my invention is:

1. A machine for the extrusion of material in a plastic state, comprising an extrusion chamber having an extrusion orice, a screw thread impelling deviceior driving plastic material into said chamber and out through the orice, said device comprising two longitudinally extending and relatively rotatable concentric parts, one of said parts being an axially movably mounted member which moves axially to a limited extent without positive restraint under the inlluence of a change in the thrust exerted upon it by the impelled material, and means actuated by axial movement of the axially movably mounted member, for controlling by heat transfer the temperature of an appropriate part of the impelling device, whereby to adjust the output of the device in a direction tending to restore it to normal. 2. A machine for the extrusion of material in a. plastic state, comprising an extrusion chamber having an extrusion orice, a screw thread impelling device for driving plastic material into said chamber and out through the orifice, said device comprising two longitudinally extending and relatively rotatable concentric parts, one of said parts being an axially movably mounted member which moves axially to a limited extent without positive restraint under the iniiuence of a change in the thrust exerted upon it by the impelled material, means for supplying heat to an appropriate part of said impelling device, and means, actuated by axial movement 0f the axially movably mounted member. due to a change in pressure in the plastic material at the delivery end of the impelling device, for regulating the supply of heat to an appropriate part of the device, in a direction to counteract the said change in pressure in the plastic material.

3. A machine for the extrusion of material in a plastic state, comprising an extrusion chamber having an extrusion oriiice, a screw thread impelling device for driving plastic material into said chamber and out through the orifice, said device comprising two longitudinally extending and relatively rotatable concentric parts, one of said parts being an axially movaby mounted member which moves axially to a. limited extent without positive restraint under the influence of a change in the thrust exerted upon it by the impelled material, and means, actuated by axial movement of the axially movably mounted member due to a change in pressure in the plastic material at the delivery end of the impelling device, for regulating the abstraction of heat from an appropriate part of the impelling device, in a direction to counteract the said change in pressure in the plastic material.

4. A machine for the extrusion of material in a plastic state, `comp-rising an extrusion chamber havingI an extrusion oriiice, a screw thread impelling device for driving plastic material into said chamber and out through the orifice, said device comprising two longitudinally extending and relatively rotatable concentric parts, one of said parts being an axially movably mounted member which moves axially to a limited extent without positive restraint under the influence of a change in the thrust exerted upon it by the impelled material, means for adjusting the output of said impelling device, and means operatively coupling said output-adjusting means with said axially movable member whereby axial movement of said axially movable member due to a change in pressure in the plastic material at the delivery end of said impelling device actuates said output-adjusting means in a direction to restore the output of the device to normal.

5. A machine for the extrusion of material in a plastic state, comprising an extrusion cham-A ber having an extrusion oriiice, a screw thread impelling device for driving plastic material into said chamber and out through the orice, said device comprising two longitudinally extending and relatively rotatable concentric parts, one of said parts being an axially movably mounted member which moves axially to a limited extent Iwithout positive 4restraint under the inuence of a change in thrust exerted upon it by the impelled material, an extension on said axially movable member projecting rearwardly of said impelling device, means for adjusting the output of said impelling device, means operatively coupling said output-adjusting means with said extension, whereby axial movement of said extension due to a change in pressure in the plastic material at the delivery end of said impelling device actuates said output-adjusting means in a direction to restore the output of the device to normal.

6. A machine for the extrusion of material in a plastic state, comprising an extrusion chamber having an extrusion orice, a pair of screw thread impelling devices for driving plastic material into said chamber and out through the orice, each device comprising two longitudinally extending and relatively rotatable concentric parts, the two devices being in axial alignment with their delivery ends together and a rotatable part of one device being united to the corresponding part of the other device and said united parts constituting an axially movably mounted member which moves axially to a limited extent without positive restraint under the inuence of a change in the resultant axial thrust exerted thereon by the impelled material, means for adjusting the outputs of the impelling devices, said means being operable diierentially on the two impelling devices, and means operatively coupling said output-adjusting means to said axially movably mounted member, whereby axial movement of said axially movable member due to a change in the resultant axial thrust exerted thereon operates said output-adjusting means in a direction to reduce the resultant axial thrust to normal.

, 7. A machine for the extrusion of material in a plastic state, comprising an extrusion chamber having an extrusion orii'lce, a pair of axially aligned screw thread impelling devices for driving plastic material into said chamber and out through the orifice, each device comprising two longitudinally extending and relatively rotatable concentric parts, an axially movably mounted, rotatably driven member carrying a pair of screw threads of opposite hands constituting parts of said impelling devices, a pair of thrust washers, between which said driven member floats axially with respect to the other parts of said impelling devices concentric with said driven member, means for adjusting the outputs of the impelling devices, said means being operable dilerentially on the two impelling devices, and means operatively coupling said output-adjusting means with a part of said driven member outside said impelling devices, whereby axial movement of said driven member due to a change in the resultant axial thrust exerted thereon operates said output adjusting means in a direction to reduce the resultant axial thrust to normal.

8. A machine for the extrusion of material in a plastic state, comprising an extrusion chamber having an extrusion oriiice, a screw thread impelling device for driving plastic material into said chamber and out through the orice, said device comprising two 'longitudinally extending and relatively rotatable concentric parts, one of said parts being an axially movably mounted member which moves axially to a limited extent without positive restraint under the influence of a change in the thrust exerted upon it by the impelled material, means for adjusting the output of said impelling device, means for actuating said output-adjusting means in circumferential sliding engagement with said axially movable member and responsive to axial movement thereof, whereby axial movement of said axially movable member due to a change in the pressure of the plastic material at the delivery end of said impelling device actuates said outputadjusting means in a direction to restore the output of the impelling device to normal.

FRANCIS TADMAN GRIFFITHS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,533,191 Kaiser et al. Apr. 14, 1925 869,307r Jones Oct. 29, 1907 2,076,200 Horley Apr. 6, 1937 2,172,651 Dunsheath Sept. 12, 1939 FOREIGN PATENTS Number Country Date 656,032 Germany Jan. 28, 1938 24,230 Austria May 10, 1906